462 CHEMICAL ENGINEERING TABLE 18.—TRUE vs. APPARENT TEMPERATURES MEASURED BY RADIATION PYROMETERS WHEN SIGHTED UPOM MATERIALS IN THE OPEN Observed temperature, degrees Centigrade True temperature, degrees Centigrade Molten iron Molten copper Copper oxide Iron oxide Nickel oxide 600 650 700 750 800 850 900 950 1,000 1,050 1,100 1,150 1,200 1,130 1,210 1,290 720 775 830 890 945 1,000 1,060 1,115 1,170 630 710 755 800 845 . 895 940 985 1,030 1,075 1,120 1,165 1,210 1,255 735 1,200 1,270 1,340 1,410 1,475 1,550 1,610 1,680 1,750 840 945 1,050 1,155 ...... 1,260 Resistance Thermometry.—The operation of a resistance thermometer depends upon the variation with temperature of the resistance (mass resistivity) of an electrical conductor. A high temperature coefficient of resistivity might accordingly appear of primary importance. Certain oxides possess this property, having a resistance at room temperature from a hundred thousand to a million times that at 1,000°C. However, the resistance of a suitably mounted material may be easily and accurately determined so that the consideration of high temperature coefficient is rarely, if ever, the deciding factor in the choice of the material. The fundamentally important property is reproducibility and constancy of resistance. A further desirable feature is a simple and convenient relation between temperature and resistance. Platinum of the highest obtainable purity meets these requirements up to about 1,100°C. The temperature coefficient of resistance of platinum is so well established that the working temperature scale up to 1,083°C. is based upon this form of pyrometer calibrated according to certain specifications. On the basis of this reproducible working temperature scale, temperatures may be measured to about 0.1°C. at 1,000°C. To obtain such precision, however, requires accurately constructed apparatus and very careful experimental manipulation. For industrial purposes, resistance thermometers are made usually either of platinum or of nickel, the latter material being fairly satisfactory for temperatures below 300°C. In the form of platinum thermometer made by Leeds & Northrup Co. about 40 cm. of fine wire is wound upon a mica frame into a spiral coil about 4 cm. in length and 0.7 cm. in diameter. The length of the wire is so adjusted that the resistance of the coil is about 8.3 ohms at 0°C. The frame is made of crossed strips of thin mica notched at the edges to hold the wire in place. From the ends of this coil lead wires of platinum or gold are carried to the terminal head of the thermometer. The lead wires are insulated and held apart by mica discs through which the wires are threaded.